Usually, the surface of a bone is covered with periosteum. The inside of the periosteum consists of bone substance (compact substance and spongy substance). In the bone substance, there is an inner cavity filled with bone marrow, which is called “medullary cavity.” Various cells are present in the bone marrow, and these cells are collectively referred to as “bone marrow cells.”
The bone marrow is known to be a hematopoietic organ. The bone marrow cells include mature and immature blood cells and hematopoietic stem cells. Thus, the bone marrow is collected from the bones of laboratory animals for the purpose of immunological studies and toxicity tests. In the case of large animals such as dogs and monkeys, the bone marrow can be directly collected from the bone of a subject under anesthesia using a bone marrow needle. On the other hand, in the case of small animals such as rats and mice, the bone marrow can be collected by extracting the femur from a subject, excising both the ends (epiphyses) of bones, injecting a liquid such as fetal bovine serum (FBS) into the bone from the cut surface using a syringe having an injection needle attached thereto so as to wash out the bone marrow (refer to, for example, Kakiuchi S. et. al., J. Toxicol. Sci., vol. 29, No. 2, p.p. 101-111, 2004).
The present inventors have applied the above method of collecting cells in a bone by injecting a liquid into the interior of the bone to bones extracted from large animals. That is, the present inventors have made two holes spaced in a longitudinal direction in the femur extracted from a pig, introduced physiological saline from one of the holes, and tried to collect a cell-containing liquid from the other hole. Surprisingly, the cell-containing liquid has been hardly collected from the hole, and most of the liquid has leaked from the surface of the bone. In this manner, the present inventors have found that, in the method of collecting cells by introducing a liquid into the interior of a bone, the liquid leaks from the surface of the bone, whereby the cells in the bone cannot be collected at a high yield.